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The remainder of the sulphur unites to the potash of the alum and forms sulphuret of potash. Thus the powder consists of sulphuret of potash, alumine and carbon. When this is exposed to the air the sulphuret absorbs the oxygen, not only from the air itself, but also from the water which it contains with such avidity, as to occasion a degree of heat which sets fire to charcoal and sulphur.
The nitrates by the action of fire, furnish oxygen gas Most of them scintillate when thrown on burning charcoal. >
Obs. This last effect is in consequence of their increasing the combustion by imparting oxygen gas. Hence most of the nitrates uro capsb1 <<fsupporting combustii n.
444. Nitrate of Potash. Nitre. Saltpetre. This well known substance is found native in all countries, where circumstances are favorable to its production. It however seldom occurs in large masses, but is generally found either in the form of a whitish efflorescence on the soil, or in small masses, or incrustations in limestone caverns. Its particles are also diffused through particular kinds of soi:, but never to any considerable depth.
Obs. 1 The mode of obtaining nitr e is by lixiviation, that is by passing water through, the earth which contains it. The water dissolves the nitre, which is then crystalized by-ja,a-po ration. .|F V
2". The calcareous caverns which abound in Kttmucky furnish largt quantities of ibis,salt. One of these situated in Ma;lison county, is upwards of 600 x ai ds in length and about 40 feet wide. It extends through a hill, and affords a passage for horses and waggons to bring out the earth to be lixiviated. One bushel of this earth yields from one, to two pounds of nitre.
3. The earth taken from under old and. decayed buildings, which have been inhabited, always contains a quantity of this salt, and is sometimes lixiviated for this purpose. During our revolutionary struggle, considerable quantities of nitre were manufactured in this war.
445. The formation of nitre depends on circumstances which are easily imitated by art. Hence a considerable proportion of what is used in the arts, is produced by what are called artificial nitre beds. These consist of certain materials thrown together in heaps, with attention to the circumstances which are known to favour its production.

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